The present invention relates to an acceleration sensor.
Acceleration sensors of the type having an oscillating structure which is movably suspended on a substrate as a seismic mass are known. This seismic mass is deflected in response to the action of an acceleration and changes its position relative to the substrate. The seismic mass is assigned evaluation arrangements which measure the degree of deflection due to acceleration. Conventional evaluation arrangements are, for example, piezoresistive, capacitive, or frequency analog evaluation systems. In the case of the capacitive evaluation arrangements, the seismic mass is provided with a comb structure which co-operates with a stationary comb structure, i.e., one that is connected to the substrate. Between the individual webs of the comb structures, capacitances come about whose magnitudes vary as the seismic mass is deflected.
These changes in capacitance may be measured via evaluation circuits, thus permitting detection of an acceleration acting upon the acceleration sensor.
As is generally conventional, one plane of oscillation of the oscillating structure within which the deflection due to acceleration takes place lies in a substrate plane. It is conventional to assign laterally acting stops to the oscillating structure, the intention for the stops being to prevent the comb structure which is connected to the oscillating structure from striking the stationary comb structure which is connected to the substrate. In this manner, mechanical destructions of the comb structures are prevented.
The conventional acceleration sensors have the disadvantage that an acceleration which acts in a direction essentially perpendicular to the plane of oscillation may cause the oscillating structure to deflect out of the plane of oscillation. In response to the action of a correspondingly high acceleration acting in a direction essentially perpendicular to the plane of oscillation, it is possible for the oscillating structure to jump out of the existing lateral guide structures so that the functioning of the acceleration sensor is impaired or made impossible. Since acceleration sensor of that kind are used, for example, in safety-relevant equipment of motor vehicles, for instance, for tripping air bags, belt pretensioners, or the like, a failure involves a considerable safety hazard.
The acceleration sensor according to the present invention may offer the advantage that the performance reliability of the acceleration sensor may not be impaired by accelerative forces acting in a direction essentially perpendicular to the plane of oscillation. By providing stop arrangements which limit a deflection movement of the oscillating structure in a direction essentially perpendicular to the plane of oscillation of the oscillating structure, it may be achieved in an advantageous manner that the oscillating structure may not jump out of its lateral guide structures in response to the action of generally non-detectable accelerative forces which may be in a direction essentially perpendicular to the plane of oscillation. Besides the conventional lateral guide structures, the stop arrangements thus constitute additional guide structures which act perpendicularly (vertically) to the plane of oscillation.
In an example embodiment of the present invention, provision may be made for the vertically acting stop arrangements to be arranged underneath the oscillating structure. In this manner, it may be achieved that these stop arrangements are integrated in the acceleration sensor so that no additional device height may be needed. The stop arrangements may be connected to the oscillating structure with a force-locking fit; a counterstop being formed by an element which may be connected, with a force-locking fit, to the substrate. In this manner, it may be possible for the stop arrangements to be integrated in the sensor element in a simple manner. Moreover, the element constituting the counterstop may be an evaluation electrode of the evaluation arrangements, the evaluation electrode being connected to the substrate. In may be advantageously possible to maintain a conventional and proven efficient, acceleration sensor layout so that the outlay for manufacturing the acceleration sensors including the additional, vertically acting stop arrangements is essentially not increased.